The present invention relates to an electrophotosensitive material for use in electrostatic process copying machine, laser beam printer or the like.
In an image forming apparatus making use of electrophotographic technology such as electrostatic process copying machine, an electrophotosensitive material forming a photosensitive layer on a substrate having a conductive property is employed.
When the electrophotosensitive material is exposed to light with the surface being charged positively or negatively, the illuminated portion is electrically charged, that is, a hole and an electron are formed, one of them is transferred up to the surface of the photosensitive layer to neutralize the electric charge of the surface, while the other is transferred up to the conductive substrate to be biased. On the other hand, in the unilluminated portion, the electric charge is left over, and a pattern of the charge corresponding to the exposure image is formed on the surface of the photosensitive material, that is, an electrostatic latent image is formed.
As the electrophotosensitive material, the so-called function separate type separating the charge generating function and the charge transfer function by combining a charge generating substance for generating an electric charge by irradiation with light and a charge transfer substance for transferring the generated charge is widely employed because the sensitivity may be enhanced easily. The electrophotosensitive material of function separate type is available in the laminate type having a laminate photosensitive layer comprising a charge generating layer containing the charge generating substance and a charge transfer layer containing the charge transfer substance, which is formed on the surface of a conductive substrate, and in the single layer type having a single layer type photosensitive layer containing the charge generating substance and charge transfer substance, which is formed on the surface of a conductive substrate.
Besides, in the electrophotosensitive material of function separate type, the organic photosensitive material, of which the entire photosensitive layer of the single layer type or laminate type, is formed on the surface of the conductive substance. The photosensitive material is composed of an organic layer wherein the functional components of charge generating substance, charge transfer substance and others are contained in the binding resin alternatively the complex photosensitive material with a part of the photosensitive layer of the laminate type as an organic layer is preferably used because the margin of selection of substance is wide, productivity is excellent, and the degree of freedom of functional design is high.
On the surface of the electrophotosensitive material, moreover, a surface protective layer may be laminated in order to prevent its mechanical wear or optical deterioration.
In the conventional electrophotosensitive material, however, the apparent charge generating efficiency was low, and therefore the sensitivity was not achieved as designed. Yet, if provided with a surface protective layer, the residual potential was high, and the stability and durability were lowered when exposed repeatedly.
These problems are attributable to the fact that the charge transfer substance hitherto employed is mostly the electron donor substance (hole transfer substance) which is excellent in the hole transfer capacity but is inferior in the capacity for transferring electrons.
That is, in the state where the electrons charged by exposure are left over in the photosensitive layer without being transferred, motion of the holes is also impeded by the coulombic force of the electrons, and the rate of rebonding of electrons and holes increased, which results in lowering of the quantity of holes or electrons contributing to formation of the electrostatic latent image, that is to say, the apparent charge generating efficiency is lowered and the sensitivity of the photosensitive material drops. Moreover, when a large quantity of electrons is left over, the residual potential of the photosensitive material is heightened by their accumulation.
It has been recently studied, accordingly, to contain an electron receptor substance (electron transfer substance) such as diphenoquinone derivative expressed in formula (II) in the photosensitive layer or surface protective layer: ##STR2## where R.sup.2, R.sup.3, R.sup.4, R.sup.5 are the same or different alkyl groups.
However, hitherto used electron transfer substances such as diphenoquinone derivatives above are (1) inferior in compatibility with binding resin and others, and (2) characterized by such a color as to impede transmission of light, and hence a large portion could not be added to the photosensitive layer or surface protective layer. The conventional photosensitive material, therefore, could not sufficiently exhibit the electron transfer capability, and the foregoing problems have not been completely solved.